A Long Term Evolution-Advanced (LTE-Advanced) system is provided with greatly improved peak rates, and carrier aggregation has been introduced to the LTE-A system for better compatibility with an LTE system.
Unlike a User Equipment (UE) operating over a single carrier in the LTE system, the use of carrier aggregation can have the UE operate concurrently over a plurality of uplink and downlink component carriers. An E-UTRAN NodeB (eNB) can manage and schedule the plurality of component carriers to have the UE switch between the different component carriers. The respective component carriers may be consecutive or inconsecutive, and for better compatibility with the LTE system, the maximum bandwidth of each component carrier is 20 MHz, and the bandwidths of the respective component carriers may be identical or different. A plurality of types of carriers, particularly a backwards compatible carrier, a non-backwards compatible carrier and an extension carrier, may be involved in carrier aggregation.
In the LTE system, methods in which the UE judges whether a radio link fails include:
(1) The UE determines whether a T310 timer expires;
(2) The UE determines whether a random access problem indication transmitted from the Medium Access Control (MAC) layer is received when T300, T301, T304 and T311 are disabled; and
(3) The UE determines whether an indication of reaching the maximum number of retransmissions transmitted from the Radio Link Control (RLC) layer is received.
Particularly the method in which the UE judges whether a radio link fails according to whether a T310 timer expires includes:
When a number N310 of out-of-sync indications transmitted from the physical layer of the UE are received continuously at the Radio Resource Control (RRC) layer thereof and all the timers T300, T301, T304 and T311 are disabled, a T310 timer is enabled at the RRC layer of the UE to monitor for a preset judgment length of time, and if a number N311 of in-sync indications transmitted from the physical layer of the UE are received continuously at the RRC layer thereof during operation of T310, then it is judged at the RRC layer of the UE that a radio link does not fail, and the timer T310 is disabled; otherwise, failure of the radio link is judged at the RRC layer of the UE upon expiration of T310.
In the foregoing implementation, it is necessary to measure a Cell specific Reference Signal (CRS) at the physical layer of the UE to detect the quality of a downlink radio link and thereby determine whether to transmit an out-of-sync or in-sync indication to an upper layer, i.e., the RRC layer. In a Discontinuous Reception (DRX) mode, the quality of a radio link is evaluated at least once in each DRX period, and the result of calculating the quality of the radio link in the evaluation period is compared with a radio link quality parameter threshold to determine whether to transmit an out-of-sync or in-sync indication to the upper layer. In a non-DRX mode, the UE assesses the quality of a radio link at the physical layer for a specific sub-frame of each radio frame, and the result of calculating the quality of the radio link in each evaluation period is compared with a radio link quality parameter threshold to determine whether to transmit an out-of-sync or in-sync indication to the upper layer. Particularly a point of time at which the UE evaluates the quality of a radio link is referred to as an evaluation point.
In a specific process of determining whether to transmit an out-of-sync or in-sync indication to the upper layer, the UE transmits an out-of-sync indication to the upper layer when the result of calculating the quality of a radio link is below a first radio link quality parameter threshold Qout, that is, when a current channel quality is poorer than a channel quality corresponding to Qout, or the UE transmits an in-sync to the upper layer when the result of calculating the quality of the radio link is above a second radio link quality parameter threshold Qin, that is, when the current channel quality is better than a channel quality corresponding to Qin. The out-of-sync indication corresponds to an evaluation period of 200 ms, and the in-sync indication corresponds to an evaluation period of 100 ms. Particularly the first radio link quality parameter threshold Qout and the second radio link quality parameter threshold Qin are not configured values but resulting statistics of a Block Error Rate (BER) corresponding to the quality of a downlink radio link being above 10% and below 2% under the parameter condition of a specific bandwidth, etc., specified in the protocol 3GPP TS 36.133.
In summary, since a UE operates over a single carrier in the LTE system, whether a radio link of the single carrier fails can be judged simply by determining the quality of the radio link; however, in the LTE-A system with carrier aggregation, a deteriorated-quality radio link of a carrier may not imply that a UE can not perform normal transmission of data. Therefore a method for judging radio link failure in a multi-carrier operating context is desired in the LTE-A system.